Supplemental Financial Information - Operations (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	$ 5	$ 4
Variable Lease Income	69	52
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	47	54
Net realized income on NDT funds - Non-regulatory agreement units	82	54
Net unrealized (losses) gains on NDT funds - Regulatory agreement units	(932)	379
Net unrealized (losses) gains on NDT funds - Non-regulatory agreement units	(706)	280
Regulatory offset to NDT fund-related activities	709	(348)
Decommissioning-related activities	(800)	419
AFUDC - Equity	23	22
Non-service net periodic benefit cost	10	5
Exelon Generation Co L L C [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	3	3
Variable Lease Income	69	52
Decommissioning-Related Activities [Abstract]
Net realized income on NDT funds - Regulatory agreement units	47	54
Net realized income on NDT funds - Non-regulatory agreement units	82	54
Net unrealized (losses) gains on NDT funds - Regulatory agreement units	(932)	379
Net unrealized (losses) gains on NDT funds - Non-regulatory agreement units	(706)	280
Regulatory offset to NDT fund-related activities	709	(348)
Decommissioning-related activities	(800)	419
AFUDC - Equity	0	0
Commonwealth Edison Co [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0
Variable Lease Income	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	6	5
PECO Energy Co [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0
Variable Lease Income	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	3	3
Baltimore Gas and Electric Company [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0
Variable Lease Income	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	5	5
Pepco Holdings LLC [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	1	1
Variable Lease Income	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	9	9
Potomac Electric Power Company [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0
Variable Lease Income	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	6	6
Delmarva Power and Light Company [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	1	1
Variable Lease Income	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	1	1
Atlantic City Electric Company [Member]
Supplemental Statement of Operations Information [Line Items]
Operating Lease Income	0	0
Variable Lease Income	0	0
Decommissioning-Related Activities [Abstract]
AFUDC - Equity	$ 2	$ 2